After playing supporting roles in Christopher Nolan films from The Dark Knight to Dunkirk, Cillian Murphy has reached main character status in Oppenheimer, a biopic about the father of the atomic bomb. As played by the actor, VF’s Richard Lawson writes in his review of the film, “Oppenheimer is a commanding, eerie figure—haughty and saturnine, haunted and consumed.”

As one might expect, transforming into the explosive character was not for the faint of heart. The actor started by diving into the “essential” and “highly accessible” book American Prometheus: Triumph and Tragedy of a Genius, by Kai Bird and Martin J. Sherwin, as he told VF France. Murphy’s transformation process then included reading the Bhagavad Gita, a 700-verse Hindu religious text that Oppenheimer would quote from; smoking fake cigarettes (the physicist died of throat cancer in 1967); and “skipping meals” in order to emulate the character’s rail-thin frame, he told NME.

On the Oppenheimer press tour, completed prior to the actors strike, Murphy’s costars have also detailed some of his intense methods. Emily Blunt, who plays Oppenheimer’s wife, Kitty, told People that Murphy would forego cast dinners because of the “monumental” weight on his shoulders. “Of course he didn’t want to [eat] with us,” she told the outlet. Matt Damon, who plays General Groves, added, “His brain was just too full.” Murphy confirmed this to NME, explaining, “I didn’t go out much. I didn’t socialize much, mainly because of the amount of work I had to do…. I became so immersed in the role.”

While isolated, Murphy followed intense dietary restrictions to transform into a man who, as he puts it, “existed on martinis and cigarettes.” As Blunt told Extra, “He had such a monumental undertaking. And he could only eat, like, an almond every day. He was so emaciated.” Murphy told The Guardian that “you become competitive with yourself a little bit, which is not healthy,” adding, “I don’t advise it.” But when pressed about which foods a dietitian advised him to eat or just how much weight he lost for the role, Murphy maintained his discretion: “I don’t want it [the headlines] to be, ‘Cillian lost X weight for the part.’”

In his latest interview, Murphy downplayed the impact these changes had on his real life. “You always have to take a holiday after a job,” he said. “It’s not because…as some journalists like to think, you’re a method actor or whatever. It’s because you give so much time to the job and then, suddenly, you stop. You have all this displaced energy, you know, so you kind of don’t know what to do with yourself…. But I’m a very easygoing sort of person. It doesn’t weigh me down.”